Photographing sometimes takes place utilizing a strobe flash, but in most cases objects to be photographed are irradiated with the sunlight under the open sky. The luminance of sunlight is too intense compared with those of the other various light sources and would saturate an amplifier if it is directly input to a rangefinder circuit, making it impossible to obtain data necessary for range-finding. To overcome such inconvenience, a high pass filter has conventionally been used to eliminate monotonous luminance such as that of the sunlight. However, this countermeasure disadvantageously requires a high pass filter for each amplifier and consequently makes the relevant circuit arrangement correspondingly bulky.
FIG. 9 in the attached drawings is a block diagram illustrating the extrinsic light eliminator of the prior art, in which an infrared emitting diode (IRED) is used as the light emitter 1 and signal light projected, then reflected on the object is detected by the photodetector 2 as the corresponding photocurrent. This photocurrent is converted to the corresponding voltage and simultaneously DC component and/or low frequency component possibly accompanying the photocurrent is eliminated in a current/voltage converting amplifier 3. The voltage thus processed is amplified by a gain circuit 4, then held and further amplified by a hold circuit 5. The voltage held by the hold circuit 5 is amplified again and integrated by an integrating circuit 6. A comparator 7 compares the integrated voltage with a reference voltage and, when the integrated voltage exceeds the reference voltage, applies IRED drive terminating signal to a CpU 8. With this signal, IRED drive circuit 9 and therefore the light emitter 1 are disabled.
However, with this extrinsic light eliminator of the prior art as has been mentioned above, the low frequency component or the other undesirable components cannot be adequately eliminated and the relevant circuit arrangement remains bulky.
FIG. 10 exemplarily illustrates the extrinsic light eliminator of the prior art and FIG. 11 shows its frequency characteristic. As will be apparent from FIG. 11, the DC component is not adequately eliminated. FIG. 12 is a diagram of input/output waveforms exhibited by the conventional extrinsic light eliminator, showing (a) the input current waveform derived from a signal light which is projected from the light emitter, then reflected on the object irradiated with rays coming from a tungsten filament lamp and detected by the photodetector as the corresponding photocurrent and (b) the output voltage waveform from which the extrinsic light components have been eliminated by the current/voltage converting amplifier, respectively. The input current waveform detected by the photodetector 2 consists of the extrinsic light component waveform 2a and the signal component waveform superimposed thereupon and as indicated by FIG. 12(b), the low frequency component 2b corresponding to the extrinsic light component derived from the tungsten filament lamp is imperfectly eliminated.
Such residual low frequency component 2b remains until it is processed by the integrating circuit 6 and, in a disadvantageous consequence, the data required to determine a distance to the object necessarily contains an indeterminate factor resulting in a low reliability.
In view of such situation, it is a principal object of this invention to provide an extrinsic light eliminator for an autofocusing circuit so improved that rangefinding data of high reliability can be obtained by eliminating the extrinsic light components as perfectly as possible and the circuit arrangement can be simplified by minimizing the number of parts.